This invention relates to a method for manufacturing a semiconductor device. More specifically, the present invention relates to a method for manufacturing a charge-coupled device (CCD) in which impurity regions, having a concentration gradient aligned with the transfer direction of electric charges, is formed.
In a conventional CCD, an embedded channel, which is a region for transferring the charges, is formed in a semiconductor substrate, and a plurality of transfer electrodes extending in directions crossing the transfer direction of the charges are arranged at prescribed intervals on a surface of the channel. In the embedded channel under each transfer electrode, impurity regions are formed, each having a concentration gradient forming an energy potential which deepens as it progresses toward the transfer direction of the charges. Therefore, the charges are always localized in the regions at a deeper potential, thereby the charge transfer at high speed and high efficiency may be achieved.
With respect to a method for manufacturing the impurity regions having such a concentration gradient, the following method has been known. With the method, a resist film is softened through a reflow process forming a mask having a gradient providing edge by utilizing the surface tension of the softened resist film, and impurities are injected into the semiconductor substrate through the mask. In such way, the concentration of a single impurities having the desired gradient is provided with the impurity injection (refer to a patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 5-267206).
However, the conventional method for manufacturing the CCD poses the following problem.
That is, since it is formed through the surface tension in the reflow process, the shape of the resist film forming the gradient is always upward convex. Further, it has been impossible to form the resist film having the gradient over a longer length, since only the end portions of the resist film may provide the gradient. As a result, it has been impossible to form the impurity region having the concentration gradient over the longer length. In this way, the foregoing method for manufacturing the impurity regions restricts the structure of the resist film mask to be formed, and may not form impurity regions having the various desired concentration profiles.